The number of countries was mentioned earlier on Tuesday, as Washington announced a penalty waiver for Japan and 10 EU counties which complied with American demands and reduced their purchases. However, the names of the countries were not given.

Four of the countries on America’s anger list are among top 100 buyers of Iranian crude. They are China, India, South Korea and South Africa, with the first two being the two largest buyers.

US President Barack Obama may order banks based in those countries and involved in oil trade with Iran to be cut off America’s financial system. On the other hand, he may exempt some of them from sanctions, if US national security demands it. So America’s allies like South Korea or Pakistan may dodge repercussions.

Meanwhile China on Thursday slammed America’s pressure on Iran’s trading partners following the exemption of Japan and European countries from penalties. The People’s Daily, the official newspaper of the ruling Communist Party, called the US policy misguided and selfish.

"One stand-out feature of unilateralism is this: that one's own rules become the world's rules. Everyone must respect them, and if you don't, then you will be punished," the paper said in a commentary.

The US is campaigning to cripple Iranian oil export as part of pressuring it into stopping uranium enrichment. In January the EU joined the sanctions with a six-month grace period, while Japan adopted a policy to reduce imports from Iran. Western countries say the Islamic Republic may be trying to build a nuclear weapon under the guise of its civilian nuclear program.

Tehran rejects the allegations. Some counties including UN Security Council members Russia and China say no evidence of such intension is available and oppose economic sanctions against Iran, saying they fail to resolve the problem.

The uncertainty of the situation over Iran has resulted in world oil price gradually climbing over the months. The IMF warned this week that if the flow of crude from the country is abruptly cut, the resulting price shock would deal a serious blow to the global economy.